Mobile communication devices have become ubiquitous throughout modern life. One of the fields that has benefited enormously from the increasingly-robust connectivity is the public safety realm. In North America, the Association of Public Safety Communication Officials (APCO) Project 25 (P25) is a users group that sponsors the development of standards for digital radio communications in the public safety environment. The next generation of such systems including infrastructure and handsets (also referred to herein as subscriber units) will conform to P25 Phase 2 specifications.
P25 Phase 2 uses a TDMA physical layer protocol modulation scheme different than the legacy FDMA P25 Phase 1 modulation scheme currently in use. More specifically, P25 Phase 2 dictates that Harmonized Continuous Phase Modulation (H-CPM) is to be used as the uplink (handset to base station) modulation protocol of digital data to be transmitted. H-CPM is different from the downlink (base station to handset) modulation protocol specified for P25 Phase 2, Harmonized Differential Quadrature Phase Shift Keying (H-DQPSK), and accordingly engenders different problems and solutions.
While the H-DQPSK technique is similar to the existing modulation schemes used in Phase 1 systems, the use of H-CPM techniques add complexity for receivers. For example, H-CPM techniques are much more computationally-intensive to demodulate and thus use an increased amount of processing and power. Moreover, new receiver tracking algorithm strategies must be determined to minimize the various sources of error for H-CPM techniques.
The sources of error include timing error, frequency error, and deviation error. Timing error is a result of the difference between the assumed symbol timing at the receiver and the true symbol timing. Frequency error results from two main sources: Doppler shift due to relative motion between a transmitter and receiver, and differences in the frequencies of the oscillators of the transmitter and receiver. Radio systems based are designed to conform to specific frequency stability specifications, of which one such example for P25 Phase 1 is a frequency stability of 0.15 parts-per-million (PPM) for base stations and 1.5 PPM for handsets. At a typical frequency of 850 MHz, this corresponds to a frequency error of about 1400 Hz. Assuming that the worst case frequency error due to Doppler shift is about 150 Hz, the maximum expected frequency error becomes about 1550 Hz. Deviation error is due to variance in the components used to convert the modulating signal voltages to frequency in an FM modulator, which causes the frequency deviation of the transmitted signal to differ from the ideal deviation. Systems are also often designed to conform to a maximum deviation error, of which one example for P25 Phase 1 is a maximum deviation error of approximately ±10%.
One type of H-CPM receiver that may be used in P25 Phase 2 (and other) systems processes complex modulated signals in which both in phase (I) and quadrature phase (Q) components are present. Such receivers, unlike those that operate on discriminated output to recover the transmitted signal (e.g., P25 Phase 1 system receivers) can encounter much more difficulty in compensating for the various sources of error.
It would thus be beneficial to enable receivers in P25 Phase 2 and other systems to compensate for deviation error in a received signal.
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